A normal labor process is divided into three stages. Among these stages, the first and second stages are the crucial ones which are directly involved in the delivery of fetus. The first stage of labor begins with the onset of rhythmic uterine contraction and ends at the complete dilation of the cervix which is about 10 cm in diameter. The complete dilation of the cervix marks the beginning of the second stage of labor which ends immediately after the birth of the fetus. The third stage of labor extends from the birth of the baby to the complete expulsion of the placenta. The labor progress is driven by two types of labor forces. The primary force is produced by the involuntary contractions of uterine muscle. The secondary force is produced by the increase of intra-abdominal pressure through voluntary contractions of the abdominal muscles and diaphragm. These forces cause an increase of intrauterine pressure to provide a critical expulsion force on fetus.
As often seen in clinical practice, systemic analgesic drugs, epidural anesthesia and long duration of exhaustive labor all can lead to the weakening of secondary force, and sequentially to delayed labor duration or even dystocia (arrest of labor). Numerous clinical studies have correlated a prolonged labor duration and dystocia with many undesirable outcomes, such as higher rate of infant mortality, neonatal seizures and postpartum hemorrhage. To solve these serious problems, clinical instruments (forceps or vacuum suction) or cesarean section are often required to terminate labors. However, both instrumental delivery and cesarean section are far from trouble-free. While a cesarean section is basically safe, it remains a major surgical procedure. Patients who give birth by cesarean section are at much greater risk of childbirth-related illness or death than women who deliver vaginally. Also, the average cesarean birth has a length of hospital stay double that of a normal delivery and costs up to three times as much. The higher costs associated with cesarian delivery have received greater attention due to the growth of managed care. Because third party payor payments for hospital services are often a flat fee, hospitals are motivated to reduce the duration of hospital stays and the need for operating room personnel to reduce hospital costs and increase profits. Instrumental delivery also has limitations and may result in numerous complications including head and facial injuries to fetus. Therefore, it is in the best interest of both mother and fetus to prevent the incidence of prolonged duration of labor or dystocia.
One method of decreasing the incidence of prolonged labor is oxytocin infusion, which is commonly used in clinical practice to increase the primary labor force by directly inducing uterine contraction. Other pharmaceutical methods of induction are known, including the use of dinoprosten and progesterone antagonist (RU-486), however, oxytocin has been found to have the fewest adverse side effects. Clinical evidence has demonstrated that oxytocin alone can only partially solve the problem of prolonged labor and dystocia associated with epidural anesthesia. However, a high incidence of cesarean section still occurs in patients receiving epidural anesthesia in spite of a high dosage of oxytocin infusion. If the onset of labor is induced using oxytocin because spontaneous labor has not occurred, there is actually an increased cesarean risk compared with patients who labor spontaneously. Furthermore, high doses of oxytocin have been implicated in uterine tetanus and in some adverse neonatal outcomes, including fetal asphyxia. Therefore, continuous fetal monitoring is necessary when pharmaceuticals are used for uterine hyperstimulation to monitor the fetal response to labor and the uterine response to the inducing agent.
Devices directed toward assisting in delivery are disclosed in the prior art. In the apparatus of Heidenwolf (U.S. Pat. No. 2,597,637, issued May 30, 1952), an inflatable bladder is held against the woman's upper abdomen by a wide belt. Extending from the bottom of the belt is a pair of straps which, in turn, attaches to straps surrounding the upper thighs. This structure holds the belt down to prevent slippage.
In the birth-assisting pneumatic cuff of Lee (U.S. Pat. No. 5,174,281, issued Dec. 29, 1992), an inflatable bladder fits over and around the woman's abdomen and is manually inflated and deflated in coordination with the patient's voluntary muscle contractions during the second stage of labor. This device applies pressure equally to the entire abdomen.
The Chinese patent of Fei Chao (Chinese Patent No. 2198, issued in 1989) teaches an abdominal girdle which has a generally triangular bladder (to match the rough contour of the uterus) which is placed over the patient's abdomen. The bladder is inflated manually in coordination with the woman's contractions to apply a downward pressure on the abdomen, assisting in forcing the fetus downward. While the girdle itself is very effective, the manual control of the inflation/deflation may not be easily accepted by physicians who may be reluctant to rely on a device which could be easily subject to human error with serious consequences.
Related prior art may be seen in the areas of anti-G pressure suits and in inflatable tourniquets and splits. Examples of pressure suits are taught by Crosbie et al. in U.S Pat. No. 4,534,338, issued Aug. 13, 1985, and Van Patten, U.S. Pat. No. 4,736,731, issued Apr. 12, 1988. These suits inflate in response to changes in the rate of acceleration of an aircraft. Poole, et al. (U.S. Pat. No. 4,531,516, issued Jul. 30, 1985), Manes (U.S. Pat. No. 4,548,198, issued Oct. 22, 1985) and Kitchin et al. (U.S. Pat. No. 4,520,820, issued Jun. 4, 1985) teach inflatable devices for first aid applications. The latter two patents include disclosure of controllers for maintaining constant pressure, however none of these patents addresses synchronization of inflation/deflation as would be required for a labor- and delivery-assisting device.
Detection of intrauterine contractions for use with labor assisting devices, and for general monitoring of labor is commonly performed using a tocodynamometer or tocotransducer. Tocotransducers can sense uterine activity externally and non-invasively by measuring the hardness of the abdominal wall. They are held in place by a belt-like device which holds the sensor in the vicinity of the fundus (the top of the uterus). Use of such devices has been reported since the 1930's, and the general configuration of the tocotransducers has changed little since the 1950's. The guard-ring tocodynamometer which was developed in the 50's by Smyth, et al., includes a resistance strain gauge supported within a rigid ring. This device has been identified as the only pressure sensor that externally provides an absolute estimation of the intra-uterine pressure. (See, e.g., C. Sureau, et al., Chapter 61, "Electronics and Clinical Measurement" in Obstetrics and Gynaecology, Elsevier Press (U.K.),1983.) The ring of the tocotransducer is pressed against the skin to flatten it, thus providing a fixed area of contact for the strain gauge. Increases in intra-uterine or intra-abdominal pressure, depending on placement, cause the skin area within the cavity formed by the ring to press against the strain gauge, providing the pressure reading. The skin area which must be covered in order to obtain reasonable accuracy with the tocotransducer is fairly large--on the order of 50 cm.sup.2 or more, and a considerable amount of pressure must be exerted on the ring to flatten the area completely. During labor, multiple sensors are usually placed on the patient's abdomen, e.g., for measuring intra-uterine pressure, fetal heart rate and fetal movement. The belts used to hold the rigid sensors in position must be wide and tight enough to provide stability. This increases patient discomfort, and the abdominal area can quickly become covered with various monitoring devices, so that it may become difficult to place every sensor at its ideal position, detracting from the accuracy of the monitoring. Further, existing tocotransducers are sensitive to environmental factors such as humidity and temperature, requiring frequent calibration, and have limited force ranges and maximum tolerances, such that the sensors can become saturated, or even be subject to failure, if not properly positioned and tensioned against the patient's skin. For example, a commonly-used tocotransducer sold by Hewlitt-Packard has a maximum tolerance of about 1 kilogram (2.2 pounds) before failure. A similar device sold by Huntleigh has a range of approximately 100 grams and a maximum tolerance of about 320 grams (0.7 pounds).
General sensor technology has improved with developments in microelectronics, and semiconductor-based pressure sensors have been developed and made commercially available. One such solid state force sensor is marketed as the SenSym FS01 Series made by Kavlico Corporation of Moorpark, Calif. The devices are low-cost piezoresistive integrated circuits encased within a plastic housing. The integrated circuit structure and process for making the circuit are disclosed in U.S. Pat. Nos. 5,578,843 and 5,576,251, of Garabedian, et al., the disclosures which are incorporated herein by reference.
Briefly, the semiconductor sensor of Garabedian, et al. utilizes conventional MOS (metal-oxide-semiconductor) fabrication techniques to form MOGFETs (MOving Gate Field Effect Transistors) and MOPCAPs (MOving Plate CAPacitor). A flexible gate structure in the transistor spans a cavity, the bottom of which is an active region. Flexion of gate structure due to external pressure modifies the channel length to provide an electrically measurable change. In the capacitor, the top plate spans a cavity, the bottom of which is the other plate. The capacitance varies with flexion of the top plate, which variations are detected.
Another solid state pressure sensor is disclosed in U.S. Pat. No. 5,113,868, of Wise, et al. This sensor is "ultraminiature" and is designed for use medical application for use in catheters and implantable devices. Such devices are very small, intended to detect minute variations in pressure with a small range, and are costly to manufacture.